1. Field of the Invention
The present invention relates to optical scanners and in particular to a film guide for such scanners.
2. Description of the Prior Art
Optical scanners, particularly laser scanners have been successfully employed to provide a permanent record of video signals by thermal and ablative recording on media such as photographic film.
Such scanners, particularly laser scanners may be of the straight line or of the curved line type. See an article entitled "Recorders: flat vs cylindrical" by Ed Hartfield in Laser Focus, pp. 47-49, April, 1973, for a description of flat field and cylindrical surface scanners. The term "straight line" scanning is commonly referred to as flat field scanning in the field of optics, while the term "curved line" scanning refers to "circular" scanning developed on curved or cylindrical surfaces, curved relative to the focused beam. The present invention is concerned with flat field (straight line) scanning. In either type, a transverse scan of a focused laser beam, i.e., a scan across the direction of movement of the recording medium, is used to record or read information into or from the media. In all types of laser scanners the film must be positioned accurately with respect to the optics in order to maintain focus. Accurate positioning becomes more critical as the desired system resolution increases.
The film positioning mechanisms for flat field scanners are complicated by the natural tendency of the photographic roll film, to curl away from the plane of the film in a direction perpendicular to its length when unrolled, i.e., to curl along the edges. The edge curl causes substantial non-uniformities in film spacing across its width and thereby severely reduces the fidelity of recording and reading near the side edges. Edge curl control systems have been proposed as by U.S. Pat. No. 3,533,058 to Sandford Platter, entitled "Edge Curl Control for a Flat Vacuum Head," issued Oct. 6, 1970. This patent describes a system using magnetic tapes.
In flat field scanners, utilizing a cylindrical surface to carry the film to the scanning position, rollers are used to guide the film before and after the laser scanning position. The rollers are suitably spaced and tensioned to inhibit film edge curling. In such scanners light which passes through the film during recording or playback modes is reflected back onto the film by the surface of the cylinder. This back reflection causes scattered light to reduce system performance with respect to resolution, and maximum density and furthermore reduces clarity by increasing what is known in the art as "background fog." A typical system is shown in an article entitled "Advanced Laser Image Recorder" published in Applied Optics, December, 1972, pages 2778-2782.
It is further known to use a gaseous bearing for supporting the film as it moves over the scanning or processing zone. See for example, U.S. Pat. No. 3,416,148 to Donald G. Berghaus, et al., entitled "Compound Radius Transducer Head," issued Dec. 10, 1968. See also U.S. Pat. No. 3,347,447 to C. D. Rush, entitled "Tape Guidance System," issued Oct. 17, 1967. The fluid bearing of air reduces wear by eliminating mechanical contact between a tape and a transducer head.